Fluid Flow, Overpressures and Fracture Sealing Events in Mesozoic Limestones, Jebel Akhdar Dome, Oman Mountains

Hilgers, Christoph¹, David Kirschner², Jean-Paul Breton³, Janos Urai¹ (1) RWTH Aachen, Aachen, Germany (2) Saint Louis University, Saint Louis, MO (3) BRGM Oman Branch, Muscat,

Sealed fractures are frequently described to have acted as important fluid conduits, although studies considering fracture formation, vein growth history and regional scale vari­ations are rare. We studied veins hosted in autochthonous Mesozoic limestones of the Jebel Akdhar dome, Oman, using meso- and microstructural analyses and stable isotope geo­chemistry. This reservoir rock displays different sealing and overpressure events, which dis­play the complex fluid history during subsidence and exhumation. At least six fracture seal­ing events were identified in the dome. Early sets of extension veins formed at supra-hydro-static fluid pressure and low differential stresses <40 MPa during burial and are truncated by bedding parallel veins. Evidence of exhumation is given by normal faults, which contain two different phases of sealing events. Late thrusts displace the normal faults and represent a late compression phase. Samples of these different vein sets and their host rocks were analyzed in the stable isotope laboratory at Saint Louis University to provide information on fluid-rock interaction in the dome and the scale(s) of fluid movement. Oxygen isotope val­ues range from ca. 16 to 29 per mil; carbon isotope values range from 0 to +4 per mil. The initial results from approximately 120 analyses are consistent with a rock-buffered system during formation of the earliest veins, and a more open system that allowed external fluids to infiltrate the dome during normal faulting and late thrusting. Late fluid infiltration is in accordance with mechanical constraints of high differential stresses during late faulting events and suggests drained conditions.